The present invention relates to a method for reducing the encroachment of a field oxide layer which is grown during the fabrication of a MOS device on a semiconductor substrate by means of a self-aligned process.
The fabrication of MOS semiconductor devices on a silicon substrate by means of a self-aligned process includes the growth of a thick field oxide layer on the surface of the substrate between the substrate active areas in which the devices are to be fabricated. The field oxide enhances the operational isolation between the elements by suppressing various parasitic mechanisms.
In the well-known LOCOS method of MOS device fabrication, a layer of anti-oxidant material is laid down over a relatively thin pad oxide layer formed on a silicon substrate and etched into a mask which prevents the growth of field oxide over the masked active region of the substrate. When the anti-oxidant mask is in place, the field region of the substrate surface outside the antioxidant mask is first doped with field impurity and then subjected to a processing step during which a relatively thick layer of field oxide is grown.
As is known, the field oxidation step results in the encroachment of the thin end of a wedge-shaped portion of the oxide layer between the edge of the anti-oxidant mask and the underlying thin pad oxide layer. This lateral diffusion of the field oxide between the mask and the substrate is also called a "bird's-beak" region. As is known, the bird's-beak encroachment reduces the total active area available for the following lithographic process by which the active element is formed in the active region, and causes the formation of stress or stacking faults that can result in the propagation of defects in subsequently-formed layers during fabrication of the element. Further, the relatively high time-temperature product of the field oxidation process permits the field impurities to diffuse beneath the element area; this diffusion can reduce the operational voltage threshold of the fabricated device and increase the device's parasitic capacitance.
One method for reducing field oxide encroachment is to grow a relatively thick layer of anti-oxidant material to mask the active element area of the substrate. However, the thick mask distorts the crystal structure of the underlying substrate abutting the edge of the mask, which can result in the propagation of imperfections through crystals which are later grown in the same area.
Another method of resisting field oxide encroachment includes the formation of a notch in the substrate adjacent the mask edge. The notch is filled with a material forming a barrier against the growth of field oxide between the mask and the substrate. Still another method of resisting encroachment is to lay down the field oxide before definition of the active element area in the substrate and then to precisely and carefully etch the oxide to expose the active region. However this method surrenders the desirable, self-aligning aspects of the LOCOS procedure simply to counter the field oxide intrusion.
It is therefore evident that there is a need for an improvement in widely-accepted MOS fabrication procedures which will reduce the formation of the field oxide bird's-beak without introducing unnecessary complexity in the standard procedures.